Two mutant alleles of the mrl1 locus have been identified. mrl1-1 was isolated from a Robertson's Mutator stock maintained by Paul Chomet and Brenda Lowe at Dekalb Plant Genetics. mrl1-2 is a reference allele maintained by the Maize Genetics Cooperation Stock Center. RFLP linkage analysis using mrl1-1/mrl1-1 individuals of selfed progeny from outcrosses of mrl1-1/mrl1-1 to A158 placed the mrl1 locus on the short arm of chromosome 2, within 1cM of B.

mrl1/mrl1 plants exhibit partial or total loss of the lignified midrib structure in the leaf blade. In mrl1-1 plants the loss of midrib in the leaf blade occurs primarily in juvenile leaves and only occasionally in adult leaves. In mrl1-2 plants, the midrib loss is more uniform among leaves, although the first adult leaves often retain a complete midrib. Only the midribs of the leaves are affected in these alleles. The husk leaves, glumes, coleorhiza and other leaf-like structures are unaffected. The auricle is affected in leaves of both alleles which show complete loss of midrib. In wild type leaves, the auricle is visible on the abaxial side of the leaf as two wedge-shaped areas of tissue above the ligule that touch at a point at the midrib. In mrl1 leaves which have no midrib, the auricle forms an uninterrupted broad band across the leaf. The vascular pattern is also affected in mrl1 leaves. In wild type leaves, the vascular pattern in the midrib region is distinct from that found in the rest of the leaf blade. Each pair of lateral veins in most areas of the blade is separated by 10-30 minor veins (depending on the age and specific region of the leaf). In the midrib region, each lateral vein pair is normally separated by only one or two minor veins. Affected leaves in mrl1 mutant plants are slightly wider than their wild type siblings due to a larger number of correctly spaced minor veins between each pair of lateral veins in the mid region. The resulting vein pattern resembles the marginal blade region instead of the distinctive vein pattern in the midrib region.

The cell divisions that lead to the formation of the midrib in wild type leaves are initiated directly adaxial to the developing midvein. In mrl1 leaves, these cell divisions are delayed. A survey of cross-sections of mrl1 leaf primordia of varying ages suggests that there is variation in the age at which midrib initiation begins. In some mrl1 leaves, the first divisions adaxial to the midvein will be visible midway through P3 (about half a plastochron late). In other leaves, initiation of midrib development is delayed a full plastochron, or in cases of completely midribless leaves, delayed indefinitely. This correlates with the fact that there is a large variation in the final amount of midrib that is present in mrl1 leaves and suggests that the longer the delay in midrib initiation, the smaller the final midrib will be in the mature leaf. These observations imply that the midribless phenotype of mrl1 is due to a delay in the initiation of cell divisions involved in the formation of the midrib and not due to a later arrest of a normally developing midrib.

The first cell divisions of midrib formation occur directly adaxial to the midvein in wild type leaves. It is therefore possible that there may be a connection between the positioning and development of the midvein and the formation of the midrib. In wild type leaves, the midvein is initiated in the basal portion of the leaf primordium at the end of P1. In mrl1 mutant plants, there is a delay in the initiation of midvein and of subsequently initiated flanking lateral veins. The delay ranged from one half to one plastochron, such that the midvein was initiated in mid or late P2 instead of late P1. We suggest that the variable delay in vascular initiation results in a corresponding delay in the initiation of cell divisions for midrib formation adaxial to the developing midvein in late P2 and to corresponding degrees of midribless phenotype.

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